There are many industries where the sequence of fastening operations and/or the applied torque of fastening operations are critical in assembling an article of assembly. One such particular industry is the automotive seat assembly industry.
In the automotive seat assembly industry, if the fastening operation of screws on a seat frame is not performed correctly to fasten the parts of the seat together, then the assembled seat may be more prone to possible failure. Proper fastening of a screw may require a predetermined amount of torque to be applied to one or more screws or that the screws be fastened according to a predetermined sequence, or possibly both requirements. It is also necessary that all of the fastening locations be properly subject to a fastening operation and filled with a fastener.
A common requirement in the seat industry is that certain critical screws need to be fastened with a predetermined amount of torque. The amount of torque required for different screws among a seat can also sometimes be different. Screw torque requirements can be so critical for certain industries that monetary fines or disqualification of manufactured product can occur if certain critical screws that have not been properly fastened or torqued to the predetermined value.
In seat assembly operations, it is desirable to assemble a large volume of seats on an assembly line. In modem systems, this is typically accomplished with conveyor systems that carry seats held in fixtures through multiple assembly stations. Conveyor systems may be a continuously moving line whereby seats are worked-on and assembled as the seats are moving and traveling down the line, or as an intermittent stop and go system whereby seats are temporarily stopped at each station for assembly operations and then conveyed down the line to the next station. At the stations where seat parts are assembled with screws according to a predetermined torque, torque reaction arm drivers are used. Torque reaction arm drivers provide an indication of the amount of torque applied during a fastening operation.
To achieve high volume assembly and to keep conveyor lines short, typically several different screws are fastened by a single worker at a given assembly station along the line. For example, a common arrangement is a seat assembly station where several screws are installed into the seat requiring a predetermined applied torque of the same value. This system includes a mechanism that keeps a seat at a station until the desired number of torque values is achieved with the torque reaction arm that is equal to the number of screws being installed.
While the torque reaction arm is capable of providing an indication of driven torque, this type of system can be easily tricked or subject to failure. In particular, if the worker of the torque reaction arm drives the same screw twice he can accidentally provide two torque values for one screw. In repetitive work operations requiring several tasks at a single assembly station, workers can forget which screw has been properly fastened or otherwise make an accidental error in fastening the same screw twice. The result is that one or more screws have been improperly fastened despite the total number of torque values has been achieved for the station (thereby allowing release of the seat from the station for further downstream assembly).
Even without mistakes, some workers have been known to intentionally bypass or trick existing systems. In particular, there have been instances where a worker drives a screw, then reverses the same screw and then refastens that same screw at the same location to get more than one good output value at the same location to in effect trick the system. Workers have even been known to drive a screw mounted in a panel proximate the assembly station to intentionally bypass or trick the system. The cause of these problems is difficult to understand but it may include worker frustration or fatigue with respect to properly fastening screws into a seat.
One approach to reducing employee mistakes in fastening operations is to reduce the number of tasks performed at a given work station. However, this approach increases the length and cost of the assembly line and decreases worker efficiency. Another approach is to install quality control in the form of close supervision or downstream torque checking to ensure quality and accuracy of fastening operations. However, increased supervision also increases costs and decreases overall efficiency of an assembly line. There have even been instances where companies have discovered such fastening problems of a large scale level and have had to conduct massive quality control operations by manually checking the proper installation of fasteners and thousands of torque values on seats that have already been run through the line. This is both time consuming and costly.